1. Field of the Invention
The present invention relates to a method for recovering metals from waste catalysts, particularly to a method for recovering rare earth compounds, vanadium and nickel from waste vanadium-nickel catalysts.
2. Description of the Related Art
In the oil-refining industry, catalysts have been widely used in oil-cracking processes, for example fluidized catalytic cracking (FCC), or residue oil cracking (ROC).
During the cracking processes, the FCC and ROC tend to be ineffective due to the contamination and absorption of heavy metals from feedstock oil. Therefore, a great amount of waste FCC and ROC catalysts are generated, which will lead to environmental damage if proper waste-treatment procedures have not been used.
In the conventional arts, waste-treatments of the waste FCC and ROC catalysts mainly rely on the use of calcinations or landfills. It is both wasteful and risky to discard waste FCC and ROC catalysts without any further treatment. These waste FCC and ROC catalysts are rich in metals, such as vanadium, nickel or rare earth metals, which may contribute to serious pollution or result in an ecological crisis, over time.
Besides, rare earth metal is an important material in modern industries, and is generally used in nonferrous metallurgy industries, atomic energy industries, and petrochemical industries. In the conventional arts, rare earth metals are recovered from waste catalysts via a roasting-acid leaching method, by roasting the rare earth metals at 600° C. to 700° C. under the performance of sodium carbonate, acid leaching rare earth metals with sulfuric acid, and finally precipitating and isolating rare earth metals with double salts.
However, the roasting-acid leaching method is inefficient, with only 2% to 5% of rare earth metals being recovered from the waste catalysts. Also, this method requires a lot of heat and accessory materials, so that it is both less effective and less economical.
Another conventional method to recover rare earth metals disclosed in CN101705380A and entitled as “A METHOD FOR RECOVERING RARE EARTH METALS FROM RARE EARTH METALS-CONTAINING ALUMINUM-SILICON MATERIALS” uses a similar stratagem to recover rare earth metals. This method comprises the processes of roasting, oxidizing, acid leaching and precipitating the metals with double salts. Yet, this conventional method shares the same disadvantages described above. Also, the conventional method has a poor rate of recovering vanadium and nickel from the waste catalysts.
Hence, there is a need of improving the conventional technique and developing a cost-and-time saving method for recovering vanadium, nickel and rare earth metals from waste catalysts.